docs: propose OpenTUI TUI migration

[avoidwork]

Description

Proposes migrating the TUI subsystem from Ink to OpenTUI (Zig-native terminal renderer with React reconciler).

Type of Change

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactor (no functional changes)
• Performance improvement
• CI / build / tooling

Testing

All behavioral requirements from the existing tui-interface spec are preserved. No implementation changes yet -- this PR contains only the proposal artifacts (proposal, design, specs, tasks). Implementation will be done via /opsx-apply after the PR is approved.

Spec	Status
tui-interface	Unchanged -- all behavioral requirements preserved
tui-scroll-view	Delta spec: removes ink-scroll-view dependency, ADDED native ScrollBox with sticky scroll
input-cursor	Delta spec: removes blink animation, removes blinkTimeout config, keeps cursorChar
markdown-rendering	Delta spec: removes marked/marked-terminal, ADDED native <markdown> component
tui-opentui (NEW)	Defines renderer initialization, hook replacements, and cleanup requirements

Coverage

• 100% line coverage maintained

Checklist

• npm run lint passes
• Tests pass with 100% line coverage
• No forbidden patterns used
• Conventional Commit style applied

Key decisions in the design doc

• Use OpenTUI React reconciler (@opentui/react), not raw renderables API -- preserves all React state management
• Replace ink + ink-scroll-view + marked + marked-terminal -G> @opentui/core + @opentui/react
• All JSX moves from PascalCase (<Box>) to lowercase (<box>) with hex colors
• Pure utility modules (panels.js, hooks.js, messages.js, commandParser.js) unchanged
• Entry point: createCliRenderer() + createRoot() replaces <Application> wrapper
• Runtime: OpenTUI requires Bun at runtime; tooling (lint, test, coverage) stays on Node.js

Full proposal, design, specs, and task breakdown are in openspec/changes/refactor-tui-opentui/